IMD measurement

Inter-Modulation Distortion (IMD) is a report often exchanged during a PSK QSO as a figure of merit for the received signal. It is widely assumed that a very good IMD report for an idling signal is around -30db, a poor report around -20db with the worst possible at -10db.

Howard (Skip) Teller KH6TY who developed the first panoramic PSK31 transceiver and Digipan software advises that great care must be taken when attempting to measure IMD at the receiving station. The following advice was received from KH6TY:

"...IMD is the measurement of the first pair of unwanted sidebands to the desired ones, and if the signal to noise ratio is not around 26 db, you will start to measure the noise instead of the unwanted sidebands, which will be under the noise threshold. Also, if there is any distortion in the receiver, you create a false IMD reading. Whenever I measure a station's IMD, I have to be sure the S/N is good, and then I reduce the RF gain of my transceiver until the IMD stops falling and starts rising. At this point, I believe I have almost eliminated IMD caused by the receiver distorting the signal.

Many, many times, I have observed stations giving an IMD reading that is obviously way off, for reasons of background noise or receiver distortion. In some cases, I measure IMD under 26 db, whereas the station is reporting numbers in the -22 range, leading the other operator to believe his transmitter is distorting, when it is actually very clean.

On my bench, feeding a local strong signal, I routinely measure under -30 db on my own designs, and often under -36 db. Before the output stage is fitted, I will typically measure IMD under -40 to -43 db. The highest quality transceivers will usually show IMD readings of better than -30 or -32 db when signals are strong, but you seldom see such reports offered..."

KH6TY went on to recommend that operators can reliably measure their own transmitted IMD with Digipan operating in full duplex mode, in conjunction with a separate monitor receiver. A cheap or second hand receiver is quite adequate in this case.

If you normally key the transceiver via the PC, then configure Digipan to the appropriate COM port.

Connect an audio lead from the speaker or line output of the monitor receiver to the microphone input of the sound card (Use a resistive pad to ensure overdriving of the microphone input does not occur).

Enable the sound card microphone input and set the volume to near the middle of its dynamic range.

Attach a dummy load to the transceiver.

Open the Digipan Mode control and engage "IMD Measurement" which then sets the sound card to full duplex.

Transmit an idle tone at normal operating power.

Tune the monitor receiver to display the idle tone on the Digipan waterfall display.

Adjust the monitor receiver RF gain to provide an S meter reading of around 1 to 5 (may need to short the receiver antenna input to ground to adequately attenuate the input signal).

Right click on the transmitted signal trace on the waterfall.

Read the Digipan IMD figure as now measured directly from the transmitter.

Normally expect a figure of around -30db or better to confirm the transmitter is putting out a clean PSK signal.

Note: Some soundcards and some computers, and even the near field radiation of some transceivers, may produce interfering signals when trying to observe or measure IMD. These will show up as extra lines or noise on the spectral display. Tuning the monitor receiver slightly or selecting a slightly different operating frequency before attempting to measure IMD often makes it possible to avoid these interfering signals.

The following Digipan screen shot displays an IMD of -30db for a typical test undertaken by W8NUE.